Heard somewhere about the Venus transit or Mercury transit but don’t know a dime about what actually they are?
Don’t you worry because this article lights up all about the transits and the Black Drop Effect associated with it. The black drop effect is an optical phenomenon that can be viewed during a transit of Venus or in transit of Mercury. Transit of Venus occurs when planet Venus passes directly in between the Sun and some other superior planet. This way, it becomes visible against the solar disk and it blocks a small portion of the sun. When a transit takes place, Venus can be seen from Earth as a small black dot. This dot may tend to be moving across the front visible portion of Sun. The duration of such transits can be of usually several hours.

We know that the diameter of Venus is three times that of Moon and is comparatively farther from Earth. For these very reasons, Venus appears smaller and travels slowly across the face of the Sun. The Transit of Venus is considered to be the rarest among all other predictable astronomical phenomena.

The black drop effect was earlier thought to occur due to Venus’s thick atmosphere. This indeed was proved to be the first real evidence that Venus actually had an atmosphere. However, undergoing several kinds of research and advancements in the field of Astronomic sciences, it has now been concluded to be the Optical phenomenon. Additionally, it is found that the effect was caused by the combination of these 2 factors:

  • Extreme darkening of the Sun’s disk near the edge
  • The internal imperfection of the viewing apparatus


Mercury transits were the ones which are observed frequently before. The Shuckburgh telescope of the Royal Observatory, Greenwich, London was used for observing the transit of Mercury in the year 1832. It was well equipped with a micrometer. The micrometer was used to report the events as seen through the small refractor.

When Mercury was observed during its transit in May 1832, Bessel and Argelander noticed some effect in the process. This was later called the black drop effect. Both of them observed this effect with an instrument of lesser resolution.

With more accurate measurements, a black drop effect was observed again from outside of Earth’s atmosphere during the 1999 and 2003 through transits of Mercury as it has no significant atmosphere of its own. In June 2004, during transit of Venus, many observers reported that they did not see the black drop effect. Large telescopes, better optical resolutions, and darkening of a limb might have been stated as the responsible factors for the same.

By observing the transit in such combinations, the diameter of the planet was taken. The last transit of Venus was seen on 5th and 6th June 2012. This was also the last transit of Venus of the 21st century. The transit prior to this took place on 8th June 2004. The more previous one was seen in December 1874 and December 1882. According to the studies, the next transits of Venus will take place on 10th or 11th December 2117. More later, it will be seen on 8th December 2125.


In 1627, Johannes Kepler was the first person to predict the transit of Venus by predicting the event of 1631. He failed in a way that he could not explain why the transit was not visible in most of Europe. As a consequence, nobody was able to understand his prediction and thus lost relevance.

Jeremiah Horrocks made the first recorded observation of the transit of Venus from his home in Much Hoole, England on 4th December 1639. His friend, William Crabtree, also observed the same transit from Broughton, Manchester. Kepler had predicted some kind of transits in 1631 and 1761.

Venus transits are historical of great scientific importance since they were used to gain the first real estimates of the size of the Solar System and its planets. Observations, when the transit took place in 1639, provided an estimate of the size of Venus. The same observation also estimated the distance between the Sun and Earth. These observations taken at that time was more accurate than any other up to that time. The 2012 transit provided scientists with many other research opportunities as well.


A transit of Mercury across the Sun takes place when planet Mercury passes directly between the Sun and a superior planet. It then becomes visible against the solar disk like a small spot that obstructs the light. During the transit, Mercury appears as a tiny black dot moving across the face of the Sun.

Mercury transits are more frequent than Venus transits with respect to Earth. It is because Mercury is closer to the Sun and orbits faster than any other planets. Transits of Mercury usually seem to occur around May or November. The last four transits were observed in May 2003, November 2006, May 2016 & November 2019. The next transit is said to take place on November 13, 2032. A typical transit may last for several hours.


The most common observation attribute that is to be noted is the number of times the disk of Mercury appears to be in contact with the Sun. These contacts are traditionally called the 1st, 2nd, 3rd, and 4th contacts. Among these, the 2nd and 3rd contacts occur when the Mercury disk is fully on the disk of the sun. It is generally seen that the 1st and 4th contacts cannot be accurately detected. The 2nd and 3rd contacts are readily visible.

Observed contact times for the transit of mercury between 1677 and 1881 are given in S Newcomb’s “Analysis of Transits of Mercury”. Observed 2nd and 3rd contact times for transit during 1677 and 1973 are given in “Royal Greenwich Observatory Bulletin”.

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